One of the major questions in biology is how the one dimensional information encoded in an organism's DNA sequence directs the four dimensional pattern of events that lead to the complex developmental changes that occur during its life cycle. Elucidation of any aspect of this basic question, gained from studying specific well-defined developmental systems, should be of benefit for gaining a better understanding of normal, and abnormal, development in all higher eukaryotes, including man. The genetic control of sex determination in Drosophila melanogaster is an attractive system in which to study this broad question at a specific level since several of the genes that interact as parts of a regulatory hierarchy controlling sexual differentiation have been identified by genetic analyses, and some have been isolated by molecular cloning. This study focuses on the molecular characterization of one of these developmentally important regulatory genes, the transformer (tra+) locus. Aspects of this gene's structure, expression, regulation, and function will be analyzed, using molecular biology techniques including: DNA sequencing of tra+ cDNAs and genomic clones, in situ hybridization of tra+ DNA probes to RNA in tissue sections to examine its expression patterns, in vitro manipulation of the tra+ gene or the flanking sequences followed by P-mediated transformation to examine the effects of those manipulations on tra+ regulation and function. In addition, rabbit antisera directed against lac Z-tra+ fusion proteins synthesized in bacteria, will be used as antibody probes to examine the expression, distribution, and subcellular localization of the (presumed) protein product of tra+. These studies will better define the role of the tra+ gene in the sex determination pathway, and will lead to a better understanding of this developmental process.